FIG. 1 is a block diagram of a network structure of a UMTS (universal mobile telecommunications system). Referring to FIG. 1, a UMTS comprises a user equipment (UE), a UMTS terrestrial radio access network (UTRAN), and a core network (CN). The UE is also known as mobile terminal or subscriber unit.
The UTRAN includes at least one radio network sub-system (RNS). And, the RNS includes one radio network controller (RNC) and at least one base station (Node B) managed by the RNC. And, at least one or more cells exist in one Node B.
FIG. 2 is an architectural diagram of a radio interface protocol between one UE and UTRAN. Referring to FIG. 2, a radio interface protocol vertically includes a physical layer, a data link layer, and a network layer and horizontally includes a user plane for data transfer and a control plane for signaling.
The protocol layers in FIG. 2 comprises a L1 (first layer), a L2 (second layer), and a L3 (third layer) referring to three lower layers of the open system interconnection (OSI) standard model widely known in the communications systems.
The respective layers in FIG. 2 are explained as follows. The physical layer (PHY) as the first layer offers an information transfer service to an upper layer using a physical channel. The physical layer PHY is connected to a medium access control (MAC) layer above the physical layer PHY. The data are transferred between the medium access control layer MAC and the physical layer PHY via the transport channel. Moreover, the data are transferred between one physical layer of a transmitting side and the other physical layer of a receiving side via the physical channel.
The MAC layer of the second layer transfer information to a radio link control layer above the MAC layer via a logical channel. The radio link control (RLC) layer of the second layer supports reliable data transfer and is operative in segmentation and concatenation of RLC service data units received from an upper layer. Hereinafter, the service data unit will be abbreviated SDU.
A radio resource control (RRC) layer located on a lowest part of the third layer is defined in the control plane only and is associated with configuration, reconfiguration and release of radio bearers (RB) to be in charge of controlling the logical, transport and physical channels. In this case, the RB means a service offered by the second layer for the data transfer between the UE and the UTRAN. In general, the configuring of RB means defining characteristics of protocol layers and channels necessary for offering a specific service and setting their specific parameters and operational methods, respectively.
A multimedia broadcast/multicast service (MBMS) offers a streaming or background service to a plurality of UEs using a downlink MBMS bearer service. One MBMS includes at least one session, and MBMS data are transmitted to a plurality of the UEs via the MBMS bearer service during an ongoing session.
A UTRAN offers the MBMS bearer service to UEs using radio bearers. As a type of the RB used by the UTRAN, there are a point-to-point radio bearer and a point-to-multipoint radio bearer. In this case, the point-to-point radio bearer is a bi-directional radio bearer and includes a logical channel DTCH (dedicated traffic channel), a transport channel DCH (dedicated channel) and a physical channel DPCH (dedicated physical channel) or a physical channel SCCPCH (secondary common control physical channel). The point-to-multipoint radio bearer is a uni-directional downlink radio bearer.
FIG. 3 is a diagram for explaining channel mapping for MBMS. Referring to FIG. 3, a point-to-multipoint radio bearer includes a logical channel MTCH (MBMS traffic channel), a transport channel FACH (forward access channel) and a physical channel SCPCH. The logical channel MTCH is configured for each MBMS offered by one cell and is used in transmitting user-plane data of a MBMS to a plurality of UEs.
A logical channel MCCH (MBMS control channel), as shown in FIG. 3, is a point-to-multipoint downlink channel and is used in transmitting control information associated with the MBMS. The logical channel MCCH is mapped to the transport channel FACH (forward access channel), while the transport channel FACH is mapped to the physical channel SCCPCH (secondary common control physical channel). One cell has one MCCH.
The UTRAN offering the MBMS transmits MCCH information to a plurality of UEs via the MCCH channel. The MCCH information includes a notification message associated with the MBMS, i.e., an RRC message associated with the MBMS. For instance, the MCCH information includes a message that notifies MBMS information, a message that notifies point-to-multipoint radio bearer information, access information notifying that an RRC connection is requested for a specific MBMS.